A model for cellular malignant transformation was proposed by the demonstration of a type of molecule that acts as a product of an oncogene would be expected to act. These small proteins such as Epidermal Growth Factor, Nerve Growth Factor, and others act by binding to specific Growth Factor Receptors on the surface of target cells stimulating them to divide. Several new retroviruses were identified and characterized: (a) two type C viruses from Mus cervicolor, one related to the woolly monkey-gibbon ape type C virus, the other related to the type C virus from Mus musculus. (b) A new class of retrovirus from Mus cervicolor and Mus caroli which are unrelated to all previously known retroviral groups. (c) Primate type C viruses, several Papio species (baboons) and Theropithecus. (d) A group D retrovirus from Presbytis (langur monkey) related but distinct from Mason-Pfizer Monkey virus. (e) Several isolates from Saimiri (squirrel monkey). BIBLIOGRAPHIC REFERENCES: Peries, J. and Todaro, G.J.: A hemagglutination assay for the primate syncytium-forming (foamy) viruses. J. Gen. Virol. 34: 195-199, 1977. Sen, A. and Todaro, G.J.: The genome-associated specific RNA binding proteins of avian and mammalian type C viruses. Cell 10: 91-99, 1977.